U.S. patent application number 10/420662 was filed with the patent office on 2003-10-16 for method and apparatus supporting tdd/tty modulation over vocoded channels.
Invention is credited to Leung, Nikolai K. N..
Application Number | 20030196155 10/420662 |
Document ID | / |
Family ID | 27804576 |
Filed Date | 2003-10-16 |
United States Patent
Application |
20030196155 |
Kind Code |
A1 |
Leung, Nikolai K. N. |
October 16, 2003 |
Method and apparatus supporting TDD/TTY modulation over vocoded
channels
Abstract
A method, apparatus, and article of manufacture used to
encode/decode a low activity communication signal--such as a Baudot
tone--for transmission over a telecommunications system. The
telecommunications system may include any number of wireless links.
Once the system is noticed that a low activity signal needs to be
transmitted, each vocoder used in the system to encode/decode the
signal performs a unique encoding/decoding process. In one
embodiment, frames containing errors adversely affecting a signal
are delivered to the vocoder and the "soft bits" contained therein
are used to determine the original signal transmitted. In another
embodiment, encoding of the signal may include encoding the signal
using redundancy with the encoded signal being spread across
multiple vocoder frames.
Inventors: |
Leung, Nikolai K. N.;
(Arlington, VA) |
Correspondence
Address: |
Qualcomm Incorporated
Patents Department
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
27804576 |
Appl. No.: |
10/420662 |
Filed: |
April 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10420662 |
Apr 21, 2003 |
|
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09152190 |
Sep 12, 1998 |
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Current U.S.
Class: |
714/758 |
Current CPC
Class: |
H04L 1/08 20130101; H04L
1/0045 20130101; H04J 3/175 20130101; H04W 88/181 20130101 |
Class at
Publication: |
714/758 |
International
Class: |
H04L 001/00; G06F
011/00; H03M 013/00 |
Claims
What is claimed is:
1. A method comprising: receiving a low activity communication
signal; encoding the low activity communication signal in a first
frame using channel-coding redundancy, the encoding comprising
interleaving a sequence of bits representing the low activity
signal across multiple frames; and transmitting the multiple frames
to a destination.
2. The method of claim 1 wherein encoding the low activity
communication signal includes using vocoder signatures containing
sequenced bits that are spaced further apart than sequenced bits
contained within standard vocoder parameters.
3. The method of claim 1 further comprising: receiving the multiple
frames at the destination; decoding the received frames to retrieve
the sequence of bits representing the low activity communication
signal, wherein the decoding comprising retrieving the sequence of
bits representing the low activity communication signal based on
information contained in frames adjacent to the first frame if the
first frame is not received at said destination.
4. The method of claim 1 wherein the low activity communication
signal comprises a Baudot signal.
5. An apparatus comprising: an encoder to receive and encode a low
activity communication signal in a first frame, the first encoder
to interleave a sequence of bits representing the low activity
communication signal across multiple frames and to transmit the
multiple frames to a destination; and a decoder to receive the
multiple frames at the destination, the decoder to retrieve the
sequence of bits representing the low activity communication signal
based on information contained in frames adjacent to the first
frame if the first frame is not received at the destination.
6. The apparatus of claim 5 wherein encoding the low activity
communication signal includes using vocoder signatures containing
sequenced bits that are spaced further apart than sequenced bits
contained within standard vocoder parameters.
7. The apparatus of claim 5 wherein the low activity communication
signal comprises a Baudot signal.
8. A machine-readable medium comprising instructions which, when
executed by a machine, cause the machine to perform operations
including: receiving a low activity communication signal; encoding
the low activity communication signal in a first frame using
channel-coding redundancy, the encoding comprising interleaving a
sequence of bits representing the low activity signal across
multiple frames; and transmitting the multiple frames to a
destination.
9. The machine-readable medium of claim 8 wherein encoding the low
activity communication signal includes using vocoder signatures
containing sequenced bits that are spaced further apart than
sequenced bits contained within standard vocoder parameters.
10. The machine-readable medium of claim 9 further comprising:
receiving the multiple frames at the destination; decoding the
received frames to retrieve the sequence of bits representing the
low activity communication signal, wherein the decoding comprising
retrieving the sequence of bits representing the low activity
communication signal based on information contained in frames
adjacent to the first frame if the first frame is not received at
said destination.
11. The machine-readable medium of claim 8 wherein the low activity
communication signal comprises a Baudot signal.
Description
[0001] The present Application for Patent is a Divisional
Application and claims priority to patent application Ser. No.
09/152,190 entitled "Method and Apparatus Supporting TDD/TTY
Modulation over Vocoded Channels," filed Sep. 12, 1998, and
assigned to the assignee hereof and hereby expressly incorporated
by reference herein.
BACKGROUND
[0002] 1. Field
[0003] Generally, the present invention relates to the field of
telecommunication devices for the deaf (TDDs) or text telephone
yokes (TTYs). More particularly, the invention relates to
modification of standard vocoder operation to enable reliable
transport of TDD/TTY signals within a telecommunication system. The
system may include wireless links.
[0004] 2. Background
[0005] Many deaf or hearing-impaired people use communication
terminals specifically constructed and designed to enable them to
communicate over standard telephone lines. Such devices, referred
to as telecommunication devices for the deaf (TDDs) or Text
Telephone Yokes (TTYs), are collectively referred to as TTDs in
this application. Typically, TTDs include a keyboard and a display
connected to a telephone via a modem (modulator/demodulator). The
modem is built into the TDD and is either directly connected to a
telephone line or coupled by an acoustic coupler to a normal
telephone handset. TDDs are capable of transmitting information
over telephone lines by means of coded tones to other TDDs
connected at opposite ends of the telephone line through another
modem. These tones are referred to as low activity communications
because the frequency and amplitude envelopes remain relatively
constant.
[0006] The code and protocol that is in widespread conventional use
for TDD communications is an idiosyncratic one. The code set, known
as Baudot, and the communication protocol (TDD protocol) evolved
historically at a time when many telecommunication devices for the
deaf were based on mechanical or electromechanical devices rather
than electronic devices. Accordingly, the TDD protocol was
constructed for a set of constraints that no longer are relevant to
present day devices. Those constraints work to create a code
protocol and a telecommunication network of users and devices
operating under that protocol, that is somewhat antiquated.
[0007] Traditionally, TDD communications are conducted at 50 Baud
(45.5 Baud in some countries), representing a transfer of 6
characters per sec. Other protocols now available for TDD
communications incorporate higher Baud rates, such as the ASCII
(American Standard Code Information Interchange) and enhanced
Baudot protocols. Regardless, a normal TDD communication character
set consists of characters that are 5 bits long. These characters
are analogous to a letter in an alphabet where each letter
represents a word or idea. A character is grouped with overhead
information bits prior to transfer, where each group of bits to be
transferred has a duration or unit interval equal to 20
milliseconds. For example, under conventional TDD protocol, a group
of bits to be transferred comprises 8 bits: a start bit (one source
or zero bit), five bits representing the character, and at least
one and 1/2 bits marking the stop point of the transfer group. This
20 milliseconds interval is also the frame length produced by a
vocoder, discussed below, for transmitting one frame of information
in a wireless telecommunications system.
[0008] Compared to modern telecommunication systems, TDD
transmissions occur at a snail's pace. A bigger problem is that TDD
signals are substantially constant. These slow paced, monotone
signals can create havoc in digital telecommunication systems that
transmit higher activity signals at very high rates, and especially
in telecommunication systems that include wireless links. One
example of such a telecommunication system is a code division
multiple access (CDMA) system having a large number of wireless
subscriber units. Each subscriber unit has a transceiver and
communicates within the system through satellite repeaters or
terrestrial stations referred to as cells. Each cell includes a
physical plant called a base station. A cell covers a limited
geographic area and routes calls carried over subscriber units to
and from the telecommunication network via a mobile switching
center. When a subscriber moves into the geographic area of a new
cell, the routing of that subscriber's call may be eventually made
through the new cell by a process called a "handoff."
[0009] A subscriber unit, generically referred to as a cell phone,
transmits a signal that is received by a base station. The signal
is relayed to a mobile switching center that routes the signal to a
public switched telephone network (PSTN) including telephone lines
or other subscriber units. Similarly, a signal may be transmitted
from the PSTN to a subscriber unit via a base station and a mobile
switching center.
[0010] The interface between the subscriber unit and the base
station is referred to as the air interface. The telecommunications
industry association (TIA) has provided a standard for CDMA call
processing on the air interface entitled "IS-95 Mobile
Station--Base Station Compatibility Standard for Dual Mode Wideband
Spread Spectrum Cellular System." Addendum to IS-95 are provided as
Telecommunications Service Bulletins (TSB). The standard
IS-95+TSB74 includes provisions for service negotiation on the air
interface and is incorporated herein by reference.
[0011] Service negotiation is critical to successfully transmit any
communication, especially a low activity TDD communication, over a
digital telecommunication system. One problem with modern systems,
including the one described above, is that a vocoder--a device used
in the system to encode a voice or TDD analog signal into a digital
signal, and to decode a digital signal into a voice or TDD analog
signal--has difficulty in handling the substantially monotone
signal and slow speed dictated by the TDD protocol. In current
systems, a low activity communication signal such as a TDD
communication would probably be treated by the vocoder as
background noise or signal interference and be disregarded.
[0012] What is needed is an invention that can easily be integrated
into existing communication systems and that does not require an
increase in transmission power to reduce frame error rates. The
invention should be able to sense when a low activity communication
is received, or notice should be sent to the system, and be able to
reconstruct frames containing errors by reviewing adjacent frames.
Alternatively, the invention should be capable of reducing frame
error rates by invoking a protocol to be used by the vocoders
during transmission of the low activity communication signal.
[0013] The invention should be compatible with wireless
telecommunication modulation systems, such as CDMA systems,
servicing large numbers of system users. A more robust discussion
of CDMA systems and techniques used in multiple access
communication systems may be found in U.S. Pat. No. 4,901,307,
entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM
USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee
of the present invention and incorporated by reference herein.
Further, the invention should also be compatible with other
modulation systems and techniques used in other types of
communication systems, such as time division multiple access
(TDMA), frequency division multiple access (FDMA), and amplitude
modulation (AMPS) schemes.
SUMMARY
[0014] Broadly, the present invention involves the modulation of a
low activity communication by a telecommunication system using
encoded signals. More particularly, the invention concerns a
method, article of manufacture, and apparatus that uses specialized
encoding, decoding, or both, on a low activity communication signal
to minimize a transmitted signal's frame erasure rate without
increasing transmission power. The invention also provides for
decoding a low activity signal by looking at "soft bits" contained
in erred frames, or in frames adjacent to an erred frame, in an
attempt to determine the content of the original frame.
[0015] Certain disclosed embodiments of the invention provide
unique decoding methods for a TDD signal that was encoded using
standard encoding protocol. In one embodiment, the decoder may
compare a frame containing transmission errors (erred frame) with a
vocoded frame from a known TDD signal and determine the most likely
vocoded frame that was transmitted. In another embodiment, the
decoder may review adjacent frames to determine the most likely
vocoded frame that was transmitted but received in error. And
although a TDD communication is discussed throughout this
application, it should be understood that any slow or low activity
communication may be transmitted using this invention.
[0016] Another embodiment of the invention provides for decoding as
discussed above but invokes vocoder parameters that are different
from standard vocoder parameters. When a TDD signal is received,
the encoder switches to "Baudot encoding mode," notices the decoder
of the protocol change, and uses channel coding redundancy to
further improve the decoder's chances of determining the correct
TDD signal sent even if it is contained in a bad frame. This
version of the invention replaces standard vocoder parameters with
vocoder "signatures" that are better spaced apart, thus making it
easier to distinguish between tones.
[0017] Yet another version of the invention provides for encoding a
TDD signal in vocoder frames using redundancy, but doing the
encoding across numerous vocoder frames. The information is
interleaved across "N" frames so that if a frame is lost, the
decoder can extract necessary information from adjacent frames to
determine the content of the lost frame.
[0018] The invention provides its users with numerous advantages.
One advantage is that a TDD message can be transmitted using a
digital transmission medium having wireless links. Yet another
advantage is that a TDD device can be connected to a mobile device
or subscriber's unit, such as a digital cellular telephone,
connected to the telecommunications system by a wireless link. The
invention also provides a number of other advantages and benefits
that should become even more apparent after reviewing the following
detailed descriptions of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The nature, objects, and advantages of the invention will
become more apparent to those skilled in the art after considering
the following detailed description in connection with the
accompanying drawings, in which like reference numerals designate
like parts throughout, and wherein:
[0020] FIG. 1A is a block diagram of hardware components and
interconnections of a telecommunications system incorporating
wireless links in accordance with one embodiment of the
invention;
[0021] FIG. 1B is a block diagram of a vocoder capable of
implementing the present inventions encoding and decoding methods
coupled to a prior art noticing apparatus in accordance with one
embodiment of the invention;
[0022] FIG. 2 illustrates a typical prior art TDD communication
device used in accordance with one embodiment of the invention;
[0023] FIG. 3 shows a traffic channel frame format for a rate set 1
used by a variable rate vocoder; and
[0024] FIG. 4 is a flow diagram of one method aspect in accordance
with the invention.
DETAILED DESCRIPTION
[0025] FIGS. 1 through 4 illustrate examples of various method and
apparatus aspects of the present invention. For ease of
explanation, but without any limitation intended, these examples
are described in the context of a TDD communication device attached
to a digital telecommunication system incorporating wireless links,
one example of which is described below.
HARDWARE COMPONENTS AND INTERCONNECTIONS
[0026] FIG. 1A and FIG. 1B illustrate one type of
telecommunications system 100 including wireless links and a TDD
communication device (TDD) 200 as used in the present invention. As
shown in detail in FIG. 2, TDDs usually include a keyboard and a
display that are connected to a telephone via a modem
(modulator/demodulator). The modem is built into the TDD and is
either directly connected to a telephone line or coupled by an
acoustic coupler to a normal telephone handset. TDDs are capable of
transmitting information over telephone lines by means of coded
tones to other TDDs, such as TDD 102 shown in FIG. 1, connected at
opposite ends of a telephone line through another modem.
[0027] In digital telecommunications systems using wireless links,
the TDD 200 may be coupled to a subscriber unit 104 that is used in
the telecommunications system 100 to transmit received signals.
Exemplary embodiments of a subscriber unit 104 are digital signal
telephones, such as the Q-800 manufactured by QUALCOMM,
Incorporated, and commonly referred to as cell phones. The
subscribers unit 104 as shown in FIG. 1A includes a noticing
apparatus 106 communicatively coupled to circuitry of the
subscribers unit 104. A hardwire 108 may be used to connect the TDD
200 to the subscribers unit 104 via the noticing apparatus 106, or
a device port may be used. Examples of such a noticing apparatus
and device ports are disclosed in the U.S. Pat. No. 6,205,339,
entitled "METHOD AND APPARATUS FOR ESTABLISHING TDD/TTY SERVICE
OVER VOCODED CHANNELS," issued Mar. 20, 2001, assigned to the
assignee of the present invention and incorporated by reference
herein.
[0028] The device port may be configured to receive a low activity
communication device attachment such as a plug, connector, or
receiver. These items are commonly used today for connecting
telephone and computer equipment, and are readily available from
electronics suppliers. The device port interfaces with the
attachment to communicatively connect a low activity communication
device (not shown) such as the TDD 200 to the subscriber unit 104
of the telecommunications system 100. The device port allows
information to be exchanged between a low activity communication
device and the subscriber unit 104. Regardless of whether a device
port or a hard wire is used, the noticing apparatus 106 allows for
the system 100 to be noticed that a TDD signal needs to be
transmitted.
[0029] Returning to FIG. 1A, after the noticing apparatus 106
receives the low activity communication signal, the signal is
processed by the subscriber unit 104. Very basically, a signal for
transmission is created that includes the information contained in
the low activity signal. Because the telecommunications system 100
has been noticed that a low activity signal is being transmitted,
the system adapts to assure a decipherable transmission occurs. For
example, an analog signal received from the analog circuitry 228
shown in FIG. 2 normally would undergo signal or "voice" processing
including digitizing the signal, setting a transmit power level to
protect against signal fading during transmission, compressing the
signal, and filtering. These functions may be performed by the
circuitry (not shown) of the subscriber unit 104 that includes a
vocoder. Depending upon the signal received, a variable rate
vocoder--generically referred to in this application as a
vocoder--may dynamically determine and negotiate service within the
telecommunications system 100 to provide successful transmission
and decoding of the signal. This negotiation involves establishing
the values for multiple parameters, such as the rate the vocoder
should use, the transmission power, and compression technique. A
fuller discussion concerning the processing of signals for
transmission in telecommunication system may be found in the
Electronic Industry Association standard TIA/EIA/IS-95-A entitled
"Mobile Station-Based Station Compatibility Standard for Dual-Mode
Wideband Spread Spectrum Cellular Systems, referred to as "IS-95"
and incorporated by reference herein, and other transmission
standards, including standard vocoder protocol, are well known in
the art.
[0030] However, when a low activity signal is received, a vocoder
may identify the signal as either noise, a pause, or a signal not
intended to be transmitted. Simply, a vocoder doesn't know what
service to use because it cannot identify the low activity signal
received. By noticing the system 100 that a low activity signal is
being sent, the vocoder will establish the service needed to assure
the best possible transmission and decoding of the signal.
[0031] After the low activity communication signal has been
processed and the service determined, a signal may be transmitted
using an antenna 112 over a wireless link 114. The digitized signal
is received by another antenna 116 at a remote location, such as a
base station 118, and processed by base station circuitry (not
shown) including a vocoder 120. Various based station circuitry
arrangements for telecommunications systems are well known in the
art, and a further understanding may be found in TIA/EIA/IS-95-A
referenced above. By processing the signal after receipt, a low
activity signal reflecting the information contained in the
transmitted low activity signal may be delivered to the low
activity device 102 via communication link 122. A second noticing
apparatus 118 is shown coupled to the base station 106. This
provides for a low activity signal to be sent from the low activity
communication device 102 back to the TDD communication device
200.
[0032] Communication link 122 appears bifurcated to emphasize that
that the base station 118 may not be connected directly to the low
activity device 102. The base station 118 is usually connected to a
standard PSTN switching station commonly used by telephone
companies for coordination of telephone calls and the low activity
device 102 is connected to the PSTN. In another embodiment, a
second mobile station (not shown) connected to the low activity
communication device 102 may be linked to the base station 118.
Further, the telecommunication system may include mobile switching
stations as mentioned above.
[0033] Shown in FIG. 2 is a schematic block diagram of the
circuitry of a typical TDD device 200, either a standard or
enhanced TDD, operating in accordance with the present invention.
In the TDD device 200 of FIG. 2, a keyboard 202 is provided into
which the user may input data characters. The output of the
keyboard 202 is connected to a processor 204 that serves to control
the circuit elements contained in FIG. 2. Characters that are
received or transmitted by the processor 204 are also displayed on
a display 206. Optionally, the same characters received or
transmitted may be reproduced on a device such as printer 208. Some
TDD devices may not have a printer, although it is standard for
TDDs to have a visual display of some kind so that a user can see
the characters being typed and received. The keyboard 202 thus
functions as an input source of data characters to the processor
204 while either or both the display 206 and the printer 208 serve
as local destinations for the data stream characters.
[0034] The processor 204 may be connected by a suitable data and
address bus that would typically be used for this type of
application by one schooled in the art. In FIG. 2, the bus 210
connects a read only memory (ROM) 212 to a non-volatile random
access memory (NVRAM) 214. Appropriate control lines 216 and 218
are connected from the processor 204 to the ROM 212 and the NVRAM
214 providing interactive control of these units. The ROM 212 is
intended to permanently store the program that dictates the
operation of the processor 204 as well as certain data used by the
program. For example, special character strings for
machine-to-machine communication and for synchronizing two TDDs in
an enhanced operating mode may be stored. The NVRAM 214 is used as
a buffer, a floating storage place for data coming into or out of
the TDD device 200, and for storage of standard messages as entered
by the user through the keyboard 202 and intended for rapid access.
Other circuitry configurations may be used, such as combining the
microprocessor 204 with the ROM 212 and the NVRAM 214 in a single
integrated circuit.
[0035] Also connected to the processor 204 in FIG. 2 is a telephone
keypad 220 that permits the entry of telephone numbers for dialing
by the processor 204 through telecommunications system 100. A
standard telephone handset 224 rests on a cradle 226 that
incorporates a switch (not shown) indicating whether the handset
224 is in use and thus removed from the cradle 226.
[0036] The processor 204 is communicatively connected through
analog circuitry 228 to the telecommunications system 100. This
connection is shown as a hardwire connection 230, but may be any
type of connection that can communicatively link the analog
circuitry 228 with the telecommunications system 100. The analog
circuitry 228 provides a connection between the handset and the
processor 204 allowing both Baudot tones and dialing tones to be
received by the telecommunications system 100. The analog circuitry
228 provides an interface of voice information to and from the
handset 224. The analog circuitry 228 of the TDD device 200 is
connected to the telecommunication system 100 using a connector
such as the device discussed above.
[0037] Despite the specific foregoing descriptions, ordinarily
skilled artisans having the benefit of this disclosure will
recognize that the apparatus discussed above may be implemented in
a telecommunications system of different construction without
departing from the scope of the present invention. As a specific
example, multiple subscriber unit 104 may be linked to the base
station 118, or the low activity communication device 200 may be
integrated with the subscriber unit 104.
OPERATION
[0038] After a TDD signal is received, vocoders used by the system
100 during processing of the signal are noticed or detect that a
low activity signal has been received for transmission and may use
an eighth rate traffic channel frame format to transmit the signal.
However, adaptation of the following methods for quarter-to-full
rate traffic channel transmissions may be accomplished, although an
undesirable increase in transmission power will result.
[0039] FIG. 3 shows a typical variable rate vocoder frame format
for a traffic channel using a rate set 1. The variable rate vocoder
produces a frame every 20 milliseconds using Code Excited Linear
Prediction (CELP) techniques that are well known in the art. The
frames may be formatted in full, half, quarter or eighth rate
formats depending upon voice activity. If a Baudot tone is
received, the variable rate vocoder will usually detect low
activity and use the eighth rate format, assuming the standard
vocoder currently in use can detect that a signal is being sent.
Commonly, a Baudot signal will be treated as noise and generally
ignored.
[0040] Full rate refers to the fact that each bit contained in each
frame is not repeated. Half-rate refers to sending the same number
of bits per frame, but each bit is repeated once in the frame; that
is, each unique bit will appear twice in the frame. Quarter-rate
refers to each unique bit appearing four times per frame, and so
on. The more repetitively a bit of information is sent, the less
total information is sent per frame. At full rate the signal is
sent at a higher power because a given bit is sent only once. This
full rate power level is referred to as the reference power for
purposes of this application. Because bits are repeated at lower
rates, a reduced power level is used because the power for each
repeated bit is accumulated over the frame. Assuming a fixed
minimum power is used for the transmission, a full rate
transmission will contain more frame errors than would a half rate
transmission of the same information.
[0041] Typically, the power level is set based upon a selected
frame error rate (FER) for the transmitted signal as received at a
remote location, also referred to as the target of the transmitted
signal, such as the subscriber unit. A desired FER is selected
because when a low activity signal is being sent, the actual FER
increases using current methods. This selected FER range is between
a 0.1% and a 1.0% error rate, but may be greater or lesser if
necessary for preservation of the quality of the transmitted
signal. Preferably, an FER of 0.2% is desirable for low activity
signals.
[0042] In the present invention, implementing specialized encoding
and decoding techniques controls the frame error rate. In the
exceptional circumstance that the disclosed techniques fall short
of a desired FER--in this case FER being defined as the total
number of erred frames even after reconstruction of vocoder frame
information, as discussed below, has been attempted--transmission
power could also be increased. It should be realized that any
increase required would still be less than the increase required if
the present encoding/decoding techniques were not implemented.
[0043] A. Decoder Using Soft Bits
[0044] In one embodiment, when a TDD call is received, the system
100 is either noticed or detects the call type. The system 100
processes the call from TDD unit 200 for transmission using
standard processing techniques known in the art. When the frame is
received at a remote point, for example base station 118, the call
is decoded using the present invention. If a frame error has
occurred in the physical layer, that is, if the frame does not pass
the checksum as described within IS-95, the frame is still
delivered to the vocoder 120 for decoding. Delivering the erred
frame to the vocoder is currently not done in standard IS-95
implementations. Bits contained in an erred frame are referred to
as "soft bits" because they may not all be in error and information
may be gleaned from them individually to reconstruct information
contained in erred frames.
[0045] However, detecting or being noticed that a TDD call has been
received, the vocoder decoder in the present invention processes
erred frames by looking at the vocoder parameters received and
comparing these parameters against "signatures" of TDD modulation
signals or tones as seen in the vocoder parameter space. This
compares the vocoder parameters of stored vocoded TDD tones with
those received. This comparison results in a determination being
made as to which TDD signal was most likely received.
[0046] For example, suppose a vocoder representation of a Baudot
tone of "0" is represented as sixteen "0"s in sequence, and that
the representation of a Baudot tone of "1" is represented as
sixteen "1"s. The present method considers these to be
voice-parameter-space signatures. For the following examples, three
layers are identified as: 1
[0047] Assume that the vocoder decoder receives the following
parameters: 2
[0048] The decoder recognizes the baudot tone boundaries and
recognizes that the received parameters for the second baudot `0`
are closer to `0` than `1.` The decoder decides on baudot tone `0`
and modifies the suspected error bits before decoding. For the next
baudot tone `1,` the decoder recognizes that the vocoder parameters
are closer to `1` than `0` and modifies the bits accordingly. The
decoder now uses the following sequence to produce a corrected TDD
signal: 3
[0049] This example shows the error transitions to occur at a frame
boundary, which isn't always the case. If these transitions
commonly fall within a frame, another version of the invention can
be used as follows.
[0050] If the vocoder decoder receives an erred frame where the
erred bits are contained within the frame, the vocoder may look to
adjacent non-erred or "good" frames to reconstruct the erred frame.
The adjacent frame will contain a portion of the baudot tone that
was lost in the erred frame. For example, suppose the following
signal is received: 4
[0051] The vocoder parameters for the second baudot tone `0` are
too ambiguous to make an accurate decision on the tone because the
number of `0`s is almost the same as the number of `1`s in the
vocoder frame parameters. To make a better determination, the
vocoder looks at the next adjacent frame (voc frame 3) and
determines that the tone appears to continue as a `0` into this
frame. The decoder therefore decides that this is meant to be a
baudot `0` tone in the latter half of vocoder frame 2.
[0052] As shown in the flow chart of FIG. 4, after it is determined
if a low activity signal is being received in tasks 402 and 404,
the decoder continuously monitors and updates the received baudot
tone boundaries in task 408. Otherwise as in task 406, any non-low
activity signal is processed using traditional methods. If an erred
frame is received as detected in the physical layer, the frame is
assigned an indicator N and the vocoder examines the erred frame in
task 410. If a "reliable" decision concerning whether or not the
frame is a baudot `0` or `1` can be made, such as when the frame
parameters are quite distinct, then the erred frame is modified to
reflect the parameters of the decision. A reliable decision is one
that falls within a prescribed probability of obtaining the
original frame parameters. For purposes of this invention, the
desired probability would be in the range of 51% to certainty. If a
modification is made, the method returns to task 402 and determines
the next signal.
[0053] If a reliable decision cannot be made as shown in task 412,
the vocoder reviews the next adjacent frame N+1 or, alternatively,
N-1, at task 414. If this frame is good in task 416, the decision
to modify the erred frame is made in task 418 based upon the
parameters contained within frame N+1, or alternatively, frame N-1.
If neither next adjacent frame is good, then a next best reliable
decision is made based upon the parameters contained within next
adjacent frame N+1 and frame N's parameters are modified
accordingly.
[0054] B. Encoder and Decoder Using Soft Bits
[0055] The decoder implementation in this embodiment of the
invention is similar to that disclosed above. However, to further
reduce the error rate and improve upon the accuracy and reliability
of the signal decoded, the encoder also takes advantage of the
"soft bits."
[0056] When the vocoder encoder detects baudot tones are to be
sent, the encoder switches to a "baudot tone encoding mode." In
this mode the encoder decides whether the tone received for
encoding is a `0` or a `1.` The encoder then sends this decision to
the decoder using a vocoder frame, but using channel-coding
redundancy to improve the decoder's chances of determining the
proper baudot tone. Even if the decoder receives a tone in an erred
frame, it will have a greater likelihood of determining the correct
tone sent because of the forwarded decision.
[0057] In a simplified example, if the encoder detects a baudot `1`
is to be transmitted, it sends a series of 1s to the decoder. The
series may be any length, but must be sufficient so that the
decoder can operate as discussed above in section A if necessary.
This version of the invention replaces the standard vocoder
parameters with vocoder "signatures" that are better spaced apart,
thus making it easier to decide between two tones.
[0058] C. Encoder and Decoder Not Using Soft Bits
[0059] This embodiment of the invention is another version of the
methods described in sections A and B, but the decoder is not given
the soft bits from any erred vocoder frames to process.
[0060] In this case, when the vocoder encoder detects a `1` or `0`
baudot tone, the vocoder also encodes the tone in a vocoder frame
using redundancy, but the encoding may be done across many vocoder
frames. The `1`s and `0`s are interleaved across a number of frames
M so that if one frame is lost, the decoder can extract the
necessary information from adjacent frames. The following example
shows interleaving taking place across four frames, but any number
of frames could be used. Assume the encoder detects the following
baudot tones for transmission: 5
[0061] In this example, the vocoder frame parameters for each frame
are segmented where four bits represents the detected baudot tone
in a particular vocoder frame. The entire sixteen bits represents
the detected baudot tones from the last four vocoder frames:
[0062] .vertline.baudot for frame N-3 .vertline.baudot for frame
N-2 .vertline.baudot for frame N-1 .vertline.baudot for frame N
.vertline..
[0063] To account for baudot tones not corresponding to vocoder
frame boundaries, the invention uses the following four-bit
sequence where XXYY indicates that the code in the current vocoder
frame reflects a baudot code of `X` followed by a baudot code of
`Y`: 6
OTHER EMBODIMENTS
[0064] While there have been shown what are presently considered to
be preferred embodiments of the invention, it will be apparent to
those skilled in the art that various changes and modifications can
be made without departing from the scope of the invention as
defined by the appended claims.
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